The present invention relates to an improved gate oxide material, and method of forming the same, for silicon-based devices and, more particularly, to the use of rare earth oxides, such as Gd2O3 or Y2O3 (exhibiting a dielectric constant ∈ on the order of 18) to form a gate oxide having the desired insulative properties while maintaining a thickness greater than the tunneling depth of approximately 10 xc3x85.
As integrated circuit technology advances, the gate lengths of MOSFETs become increasingly smaller. In addition, the thicknesses of the gate dielectrics, typically gate oxides, become thinner and thinner. Very thin gate oxides (i.e., less than 50 xc3x85) are often necessary for sub-micron MOS devices.
As device dimensions scale down rapidly with the advance of technology, the electric field in the thin gate oxides continues to increase. Part of the consequences of such increased electric field is the increased trap generation at the oxide interface or within the thin oxides. The trap generation and the capture of channel electrons by the traps in turn leads to increased low frequency (l/f) noise and transconductance (gm) degradation. For ultra-thin gate oxides of less than 50 xc3x85, the tunneling current also becomes significant and gives rise to accelerated degradation of the device characteristics. Indeed, the xe2x80x9cthinnessxe2x80x9d of the conventional SiO2 gate oxide is now approaching the quantum tunneling limit of 10 xc3x85.
Instead of continuously attempting to reduce the SiO2 thickness of the gate oxide, several groups have attempted to find a replacement insulator with a dielectric constant (∈) substantially greater than that of SiO2 (∈=3.9), so that the dielectric thickness can then be proportionally increased (thereby reducing the chance of a tunneling current through the oxide). It is desirable that the dielectric being thermodynamically stable with respect to the silicon surface so as to prevent reactions leading to the formation of SiO2 or metal silicides at the substrate/dielectric interface during high temperature annealing operations. To date, several xe2x80x9chigh dielectricxe2x80x9d oxides have been considered (such as Al2O3, Ta2O3, TiO2), but in each case an interfacial SiO2 layer at least 10 xc3x85 thick forms during growth of the gate oxide. An alternative approach uses a relatively thin SiNy barrier layer that is first deposited on the silicon surface to prevent the native oxide growth. However, the use of the barrier layer then requires for the total xe2x80x9ceffectivexe2x80x9d oxide thickness to exceed 15 xc3x85, another unacceptable result.
Thus, a need remains in the art for a dielectric material to be used as a xe2x80x9cthinxe2x80x9d gate dielectric on silicon-based devices that prevents the formation of the native SiO2 layer, yet also exhibits an effective thickness closer to 10 xc3x85.
The need remaining in the prior art is addressed by the present invention, which relates to an improved gate oxide material, and method of forming the same, for silicon-based devices and, more particularly, to the use of rare earth oxides, such as Gd2O3 or Y2O3 (exhibiting a dielectric constant ∈ significantly greater than that of SiO2 (approximately 4), for example, on the order of 18) to form a gate oxide having the desired insulative properties while maintaining a thickness greater than the tunneling depth of approximately 10 xc3x85.
Films of Gd2O3 or Y2O3 are grown, in accordance with the present invention, on a xe2x80x9ccleanxe2x80x9d silicon substrate surface, using an ultrahigh vacuum (UHV) vapor deposition process. It has been found that by limiting the oxygen partial pressure to less than 10xe2x88x927 during growth, oxidation of the silicon substrate surface is completely avoided. Both epitaxial and amorphous films have been found to form an oxide with the desired high dielectric constant characteristic.
In accordance with the present invention, a vicinal Si(100) substrate is preferably used, so as to promote the formation of single domain, (110)-oriented Gd2O3 or Y2O3 films. In a preferred embodiment a 4xc2x0 miscut substrate may be used.
A post-process gas anneal process may also be used to improve the leakage current density from a value of, for example, 10xe2x88x921 A/cm2 to 10xe2x88x925 A/cm2 at 1V for a Gd2O3 layer at an equivalent SiO2 thickness of 19 xc3x85.
Other and further aspects of the present invention will become apparent during the following discussion and by reference to the accompanying drawings.